Electrical measuring system



July 2, 1935. E. PETERSON ELECTRICAL MEASURING SYSTEM Filed May 26, 1954 Rad H $n I INVENTOR E. PETERSON 1mm W ATTORNEY Patented July 2, 1935 UNITED STATES QFFICE ELECTRICAL MEASURING SYSTEM Eugene Peterson, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 26, 1934, Serial No. 727,611

6 Glaims. (Cl. 179-17531) This invention relates to measuring systems, over the frequency range constitutes the transfer for example, electrical wave measuring systems. factor polar diagram.

An object of the invention is to determine wave To provide the unidirectional potentials proproperties, as for example, to measure wave portional to the in-phase and to the quadrature amplitude ratios or phase differences, or both. components respectively, use is made of the fact 5 It is also an object of the invention to measure that the output voltage of a vacuum tube watttransmission properties of wave propagating meter of the type disclosed, for example, in my paths, as for example, to measure phase shift or Patent 1,586,533, June 1, 1926, is a steady comattenuation, or both, in electric wave paths. ponent or unidirectional voltage proportional to 10 A further object is to facilitate rapid measurethe product of the two input voltages multiplied 10 ment of such quantities. by the cosine of the angle between them. (The Attenuation is used herein as generic to lo s wattmeter disclosed in that patent is a balanced, and gain, gain being viewed as negative loss. conjugate input modulator in which the two in- It is also an object of the invention to deteruts are of the same frequency.) If one of the mine transfer constant polar diagrams visually. two input voltages is also the input voltage for 15 h transmission p p rti s f a' lin or the amplifier or circuit under test and the other Work are Ordinarily P n d in th form of is the output voltage from that test circuit, then curves of attenuations and phase shift plotted as th frequency is varied, with constant input a n t fr q cy- Th p lar dia Permits voltagefor the test circuit, the wattmeter outthese two characteristics to be represented (for t voltage will be proportional to the real part 20 example as in yq Patent 1,915,440, June of the transfer factor. Now if in a second such 27, 1933) by a Single Curve Which the real wattmeter we have one input voltage (provided P Of the transfer factor is p otted aga n for example bya two-phase oscillator constructed t imaginary pa With qu y a a Daram" in accordance with the present invention as diseter. The polar diagram is especially useful in closed hereinafter) a, voltage equal in amplit 25 determining singing conditions in feedback amand frequency to the test circuit inp Voltage plifiers (for example the stabilized feedback amt ifl i phase by ninety degrees, an the p described in the yq pa J' other input against the output from the test cirtioned in Blacks P p 011 Stabilized cuit, then the resulting wattmeter output voltage Feedback Amplifiers, published in Electrical will be te dy component or unidirectional 30 Engineering, January 1934, pages 114 to- 120). vgltage roportional to the imaginary part of the In one Specific aspect the invention is a Visual transfer factor. Thus the output voltages of the method or System for measuring the transfer two wattmeter circuits are two voltages measurfactor of a circuit over a desired frequency range, able, fo example by a vacuum tube vol 5 in which the transfer factor polar diagram is two direct current meters as voltages proporobtained on the screen of a cathode ray oscillotional, by th same proportionality constant, to graph. The diagram can be obtained rapidly. the real and the imaginary components of the For example, points determining the transfer fact n fer factor, (Though the proportionality tor over a frequency range extending for inf to is here termed roportionality constant,

40 stance from .5 kilocycle to 15 kilooycles can be in ality 11-, may be a variable, for example in obtained in a Short time of the order of a minutecase the wattmeter output circuits include volume In this visual method of observation, a steady or ampntude range compressors, such for ins potential proportional to the in-phase component as 6 frequency or direct current amp fi of the transfer factor is provided in a manner whose gains are t lled by the amplit 01 pointed out below and is impressed across one the output from t t t circuit so t a the P pair of plates of a cathode-ray oscillograph, and t l t factgr ill be logarithmic instead another steady po e l proportional to the of linear.) If these two steady or unidirectional quadrature component is provided as pointed out voltages from the wattmeter output circuits be below and is impressed across the other pair of applied to the deflector plates of a cathode-ray plates, the constant of proportionality being the oscillographjthe spot produced on the screen by same for the two components. Consequently, the the beam will trace a pattern corresponding to transfer factor at any frequency appears as a the desired transfer factor polar diagram as the single point, the vector from the origin to the frequency of the input voltages for the test cirdisplaced beam constituting the transfer factor. cult and the wattmeters 1s swept over the neces- 55 The locus of all these points, i. e., vector tips, sary range.

The two-phase oscillator mentioned above is a heterodyne oscillator which supplies two voltages of phase difference and of constant amplitude and of frequency variable over the range over which it is desired to measure the transfer factor of the test circuit, as for example the range from 0.5 kilocycle to 15 kilocycles. One of the two voltages is supplied to the input of the test circuit and to the wattmeter input circuit that is connected to the input of the test circuit. The voltage of the other phase is supplied to the input of the other wattmeter circuit. duction of these two voltages, the oscillator has two independent sources each common to two push-pull modulators. One of these sources is of constant frequency, for example, kilocycles. The other is variable in frequency (yet practically constant in amplitude) over the frequency range, say from 100 to kilocycles. The variable frequency source is connected to the common branch of the input circuits of each modulator. The fixed frequency source is com nected in series with the grid circuits of each of the modulators. Phase shifting means, 'as for example resistance-capacity networks, are provided in the circuits of the fixed frequency source to produce a phase difference of 90 between the voltage that this source supplies to one of the modulators and the voltage that this source supplies to the other modulator. Consequently, there is a 90 phase difference between the beat frequency output of the one modulator and that of the other modulator, and this phase difference is preserved in the outputs of the two modulators when the frequency of the variable frequency oscillator is varied over the range from about 100.5 to 130 kilocycles. The outputs of the two modulators are the two voltages supplied, as indicated above, one to the input of the test circuit and to the wattmeter input circuit that is connected to the input of the .test circuit, and the other to the input of the other wattmeter circuit.

Other objects and aspects of the invention will be apparent from the following description and claims.

The single figure of the drawing shows a measuring circuit embodying the specific aspect of the invention referred to above.

In the drawing, a heterodyne oscillator I comprises a fixed frequency voltage source 2, a variable frequency voltage source 3, and push-pull modulators i and 4' fed from the oscillators to produce in the modulator output circuits two voltages of variable frequency and constant and equal amplitude, in quadrature phase relation, all as referred to above.

The modulators may be regarded as including low-pass filters 5 and 5' and amplifiers 6 and b. The filters attenuate any components of the primary frequencies that might pass through the,

modulators. The amplifiers may be of the feedback stabilized type referred to above, with a large amount of feedback that reduces both the variation in gain and phase shift to negligible proportions and at the same time reduces any harmonic production in the amplifiers to a point at which it is negligible compared to the harmonies existing in the outputs of the primary sources 2 and 3. Y

The sources 2 and 3 may be vacuum tube oscillators, the oscillator 2 being set for a'fixed frequency, for example 100 kilocycles, and the oscillator 3 having its frequency adjustable, for example over the range from 100 to 130 kilocycles. The output voltages of these oscillators 2 For the proand 3 are delivered from directly across their tuned circuits (not shown), thus reducing the amplitude of harmonics present in their output waves.

The variable frequency voltage from oscillator 3 is fed into the mid-branch of the input circuits of each of the conjugate input, balanced modulators 4 and 4, thus reducing to a minimum the number of variable frequency unwanted products appearing in the modulator outputs.

The fixed frequency voltage from oscillator 2 is fed through a phase splitting network comprising capacities I and I and resistances 8 and 8 to the modulator inputs. The phase splitting network, being connected in the output of the fixed frequency oscillator, is required to operate at only a single frequency. It can, therefore, consist of a simple capacity resistance circuit. The elements of the network may be variable around computed values, for adjustment by trial in the final circuit until exact phase quadrature and amplitude equality of voltages is obtained. The effects of higher order modulation products, tube capacities and slight departures of the primary frequency from its design value are thus automatically taken into account.

The two-phase heterodyne oscillator supplies voltages which may be used, for example, in testing a circuit I0, as for instance in determining the transfer factor polar diagram of the test circuit I0 by a visual method employing a cathode ray oscillograph II and two vacuum tube wattmeters I2 and I2, all as referred toabove.

The test circuit Ill may be, for example, the feedback loop of a. stabilized feedback amplifier of the type referred to above, the polar diagram of the transfer factor around the feedback loop being desired; and then the circuit I is the cir cuit obtained by opening the loop at any point which provided convenient impedances looking in both directions from the break, and. properly terminating the loop at each side of the. break in impedances insuring that the oscillator and measuring circuit impedances connected to the input and output of the circuit 1 are equal to the output and input impedances respectively of the circuit I0 under test, in order that the hamster factor in the measuring condition may not differ significantly from that existing in the operating condition,

The common branches I3 and I3 of the input circuits of the wattmeters I2 and I2 are shown connected in parallel across the output of the circuit I0 under test, through leads I5. The output voltage of modulator circuit 4 is supplied from amplifier 6 through leads I6 to the input of test circuit I 0 and the series input terminals of wattmeter I2 in parallel. (If desired, the conjugate inputs to wattmeter I2 can be interchanged and the conjugate inputs to wattmeter I 2 can be interchanged.) The rectified output of the wattmeter I2, therefore, is proportional to the product of the test circuit input and output voltages multiplied by the cosine of the transfer factor phase angle. This potential is supplied through leads I8 to one pair of plates of oscillograph II. To the series input terminals of wattmeter I2, through leads I6, is applied the output voltage of modulator circuit 4', from amplifier 6. This voltage is equal in amplitude to the test circuit input voltage but lags 90 behind it. The rectithe product of the test circuit input and output voltages multiplied by the cosine of the transfer factor phase angle minus 90", or in other words,

proportional to the sine of the transfer factor phase angle. This voltage is supplied through leads Hi to those plates of oscillograph II which produce a deflection at right angles to that produced by the plates connected to leads It. We have then across one pair of plates of the oscillograph a steady potential proportional to the real component of the transfer factor, and across the other pair a steady potential proportional to the imaginary component of the transfer factor. These two components act upon the beam of the oscillograph to produce a deflection which in amplitude and in phase is the resultant of the two component deflections and so corresponds to the transfer factor.

What is claimed is:

1. The method which comprises producing a first, a second and a third wave, said first and third waves being equal in amplitude and being in phase quadrature to each other, producing from said first and second waves a unidirectional force having a given proportionality to the product of their amplitudes multiplied by the cosine of the phase angle between them, and producing from said third and second waves a unidirectional iorce having the same proportionality to the product of their amplitudes multiplied by the cosine of the phase angle between them.

2. A system for comparing characteristics of a first wave and a second wave, comprising means for producing a third wave of amplitude equality with said first wave but in phase quadrature -thereto, means ior producing from said first and second waves a unidirectional iorce having a given proportionality to the product of their amplitudes multiplied by the cosine of the phase angle between them, and means for producing from said third and second waves a unidirectional force having the same proportionality to the product of their amplitudes multiplied by the cosine oi the phase angle between them.

3. A system for obtaining the vector ratio of a first voltage wave and a second voltage wave, comprising means for producing a third voltage wave of amplitude equality with but in phase quadrature to said first wave, means for producing from said first and second waves a unidirectional voltage having a given proportionality to the product of their amplitudes multiplied by the cosine of the phase angle between them, means for producing from said third and second waves a unidirectional voltage having the same proportionality to the product of their amplitudes multiplied by the cosine oi the phase angle between them, and means for indicating the relative magnitudes of said unidirectional forces, said second mentioned means comprising a balanced modulator and means for supplying said first and second waves to conjugate branches, respectively, of its input circuit, and said third mentioned means comprising a balanced modulator and means for supplying said third and second waves to conjugate branches, respectively, of its input circuit.

4. A system for determining the transfer constant polar diagram of a circuit, comprising means supplying to said circuit a first voltage wave for producing in the output of said circuit a second voltage wave, means for varying the frequency of said first wave, means for producing a third voltage wave of frequency and amplitude equality with but in phase quadrature to said first wave, means for producing from said first and second waves a unidirectional voltage having a given proportionality to the product of their amplitudes multiplied by the cosine of the phase angle between them, means for producing from said third and second waves a unidirectional voltage having the same proportionality to the product of their amplitudes multiplied by thecosine of the phase angle between them, and a cathode ray oscillograph having means responsive to one of said unidirectional voltages for deflecting the oscillograph beam in one direction and means responsive to said other unidirectionai voltage for deflecting the beam in a direction at right angles to said one direction.

5. The combination with a two-phase heterodyne oscillator producing a first wave and a third wave, said waves being of equal amplitude and in quadrature phase relation, of means for comparing characteristics of said first wave and a second wave, said means comprising means for producing from said first and second waves a imidirectional voltage having a given proportionality to the product of their amplitudes multiplied by the cosine of their phase difference, and means for producing from said third and second waves a unidirectional voltage having the same proportionality to the product of their amplitudes multiplied by the cosine of their phase difi'erence.

6. A multiphase heterodyne oscillator for producing voltages of diflerent phases and variable frequency, comprising primary sources, modulators, a phase shifting network connecting one or said sources with two of said modulators, means for varying the frequency of another of said sources, and means connecting said other source to said two modulators.

EUGENE PETERSON. 

